1. Field of the Invention
The present invention relates to a thermal displacement compensation method and a thermal displacement compensation device for a machine tool.
2. Description of the Related Art
In a machine tool, a feed screw and spindle are driven by a motor, so that they are expanded by heat from the motor, frictional heat from rotating bearings, frictional heat from an engaging portion between a ball screw and nut of the feed screw, thereby causing a shift in machine position. Thus, the relative positions of a workpiece to be positioned and a tool are shifted. This shift in machine position due to heat impedes high-precision machining.
Conventionally, some techniques and structures have been used to prevent this heat-induced shift of the machine position. In one such technique, thermal displacement is simply compensated at low cost without using a temperature sensor. In another technique, a command position is compensated based on displacement and temperature detected by displacement and temperature sensors. Further, there is a preventive structure in which initial tension is applied to a feed screw to eliminate the influence of thermal expansion.
The following is a description of prior art examples relating to thermal displacement compensation.
(1) In a technique disclosed in Japanese Patent Application Laid-Open No. 2002-18677, the entire stroke of a feed shaft is divided into a plurality of sections, the position of each section of the feed shaft is detected, and an average moving speed for the section is obtained from the detected position. A thermal displacement amount for the section concerned (based on heat generation and radiation and heat transmitted from other sections adjacent to the section concerned by thermal conduction) is estimated from the average moving speed thus obtained. According to this technique, high-precision compensation can be achieved without regard to the position of the feed shaft (or for every position). Further, the thermal displacement can be precisely compensated in consideration of thermal displacements due to heat from a spindle or spindle motor, as well as heat from the feed screw. Furthermore, more accurate compensation can be achieved by modifying a heat generation coefficient in a thermal displacement amount calculation formula, based on a deviation (compensation error) between the estimated thermal displacement amount (compensation amount) and an actual machine position.
Thus, according to the technique disclosed in the patent document described above, the thermal displacement amount is estimated based on the position and speed of the feed shaft, the spindle speed, and the load of the spindle motor, whereby high-precision compensation can be achieved without regard to the feed shaft position. For higher-precision compensation, however, there is room for consideration of thermal displacements that depend on changes in ambient temperatures, such as the room temperature, cutting fluid temperature, etc. Further, the patent document described above discloses a method in which a heat generation coefficient in a thermal displacement amount computational formula is modified using a compensation error. Since the computational formula includes other coefficients (heat loss coefficient and heat conduction coefficient for calculation of heat conduction from adjacent sections), the accuracy of compensation cannot be further improved by only modifying the heat generation coefficient, in some cases. Although compensation is more accurate in a position where an actual compensation error is measured, it is not always so in other positions
(2) In a technique disclosed in Japanese Patent Application Laid-Open No. 2010-82724, a ball screw shaft thermal displacement amount (thermal displacement amount of an entire ball screw shaft including those sections of the ball screw shaft where a nut is immovable, as well as a nut-carrying portion of the screw shaft) is obtained by detecting the rotational speed of a servomotor without using a sensor.
The technique disclosed in the patent document described above, like the technique disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 2002-18677, is designed to precisely estimate and compensate a thermal displacement amount without using a sensor. Since the compensation is performed without consideration of the influences of changes in ambient temperatures, such as the room temperature, cutting fluid temperature, etc., however, accurate compensation sometimes cannot be achieved. In addition, there is no indication of a method for overcoming compensation errors, if any.
(3) In a technique disclosed in Japanese Patent Application Laid-Open No. 2007-21721, a thermal displacement amount of a ball screw is obtained by means of a sensor, and a pitch error compensation value of the ball screw is obtained based on the obtained thermal displacement amount. Specifically, the length (A) of the ball screw, pitch error compensation values (Pn) for a plurality of divisions of the overall length (stroke) of the ball screw, and positions (Dn) of the divisions are previously registered in an NC device. Then, a variation (ΔA) of the length due to thermal displacement of the ball screw is measured by a sensor, and the variation of the ball screw length for the position (Dn) of each division is obtained using the calculation formula, Dn×ΔA/A. The sum of the pitch error compensation value (Pn) for the division concerned and the variation (Dn×ΔA/A) is registered as a new pitch error compensation value in the NC device. Then, pitch error compensation is performed based on the registered new pitch error compensation value.
In the technique disclosed in the patent document described above, the calculation of the new pitch error compensation value is performed on the assumption that the ball screw is uniformly displaced without regard to the position. Normally, however, the displacement amount for each division of the ball screw varies depending on the rotational speed of the ball screw (moving speed of a nut) and its moved position. Therefore, accurate compensation based on the ball screw position cannot be achieved by this method. According to the technique disclosed in this patent document, unlike the technique disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 2002-18677, the displacement amount is detected by a position sensor. Since the distribution of thermal displacements in the individual positions of the ball screw is out of consideration in this case, however, accurate compensation cannot be achieved.
(4) A technique disclosed in Japanese Patent Application Laid-Open No. 2002-144192 is an inter-two-point displacement compensation method for a feed screw, in which a difference between two points at the opposite ends of the feed screw is obtained, and error compensation is performed for the entire stroke for which an origin shift amount and pitch error compensation amount are determined by the difference.
The technique disclosed in the patent document described above, like the technique disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 2007-21721, is based on the assumption that the distribution of thermal displacements between two points is uniform, so that accurate compensation cannot be achieved based on the ball screw position. According to the technique disclosed in this patent document, unlike the technique disclosed in the above-mentioned
Japanese Patent Application Laid-Open No. 2002-18677, the displacement amount is detected by a position sensor. Since the distribution of thermal displacements in the individual positions of the ball screw is out of consideration in this case, however, accurate compensation cannot be achieved.
(5) In a technique disclosed in Japanese Patent Application Laid-Open No. 10-138091, a compensation amount is obtained according to an approximation formula based on an average moving speed, frequency of movement, moved position, and disturbance load torque of a feed shaft. If change of the obtained compensation amount exceeds a preset value, the position or temperature is measured by a sensor, and the compensation amount is updated based on the measured value.
In the technique disclosed in the patent document described above, a thermal displacement amount measured by a sensor, not one predicted according to an approximation formula, is used as a compensation amount when the change of the compensation amount is substantial. Since coefficients of a computational formula for estimating the thermal displacement amount and the like are not designed to be modified based on the result of the sensor measurement, the measurement result is not reflected in the estimation of subsequent thermal displacement amounts, so that the accuracy of the displacement amount estimation cannot be improved.